From 02d04033e24841965816b871b89532d186206ceb Mon Sep 17 00:00:00 2001
From: Alan Edwardes <alan@alanedwardes.com>
Date: Sat, 30 Mar 2024 21:50:16 +0000
Subject: [PATCH] Removes trailing null terminator when <character-string> is
 being encoded.

---
 src/Ae.Dns.Protocol/DnsByteExtensions.cs         | 7 +++++--
 src/Ae.Dns.Protocol/Records/DnsStringResource.cs | 6 +++++-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/Ae.Dns.Protocol/DnsByteExtensions.cs b/src/Ae.Dns.Protocol/DnsByteExtensions.cs
index 561a988..239bb9a 100644
--- a/src/Ae.Dns.Protocol/DnsByteExtensions.cs
+++ b/src/Ae.Dns.Protocol/DnsByteExtensions.cs
@@ -205,14 +205,17 @@ public static void ToBytes(string value, Memory<byte> buffer, ref int offset)
             offset += 1 + length;
         }
 
-        public static void ToBytes(ReadOnlySpan<string> strings, Memory<byte> buffer, ref int offset)
+        public static void ToBytes(ReadOnlySpan<string> strings, Memory<byte> buffer, ref int offset, bool nullTerminator = true)
         {
             for (int i = 0; i < strings.Length; i++)
             {
                 ToBytes(strings[i], buffer, ref offset);
             }
 
-            buffer.Span[offset++] = 0;
+            if (nullTerminator)
+            {
+                buffer.Span[offset++] = 0;
+            }
         }
 
         public static void ToBytes(int value, Memory<byte> buffer, ref int offset)
diff --git a/src/Ae.Dns.Protocol/Records/DnsStringResource.cs b/src/Ae.Dns.Protocol/Records/DnsStringResource.cs
index c76cd22..8e8cdd4 100644
--- a/src/Ae.Dns.Protocol/Records/DnsStringResource.cs
+++ b/src/Ae.Dns.Protocol/Records/DnsStringResource.cs
@@ -35,6 +35,10 @@ public bool Equals(DnsStringResource? other)
 
         /// <summary>
         /// Describes whether this string resource can use string compression.
+        /// If this resource describes a &lt;domain-name&gt; as per RFC 1035 terminology,
+        /// then compression can be used. If it describes a &lt;character-string&gt; as per
+        /// RFC 1035 terminology, compression cannot be used. In addition, &lt;domain-name&gt;
+        /// entries end in a null terminator, whereas &lt;character-string&gt; resources do not.
         /// </summary>
         protected abstract bool CanUseCompression { get; }
 
@@ -49,7 +53,7 @@ public virtual void ReadBytes(ReadOnlyMemory<byte> bytes, ref int offset, int le
         /// <inheritdoc/>
         public virtual void WriteBytes(Memory<byte> bytes, ref int offset)
         {
-            DnsByteExtensions.ToBytes(Entries.ToArray(), bytes, ref offset);
+            DnsByteExtensions.ToBytes(Entries.ToArray(), bytes, ref offset, !CanUseCompression);
         }
     }
 }